Method and apparatus for froth flotation

ABSTRACT

A process of separating a desired constituent from a mixture of particulate matter including the steps of: conditioning a liquid mixture of particulate matter with a frothing agent to create a pulp; aerating the pulp to generate a float fraction of froth supported on the surface of a non-float fraction of pulp; separating a portion of froth from the float fraction; draining the separated froth; washing the separated froth with a liquid to dislodge particles comprising one or more of non-selectively attached, entrained, and entrapped particles; and recovering the washed froth, is disclosed herein. Also disclosed herein are a froth cleaning apparatus and a froth flotation apparatus for separating a desired constituent from a mixture of particulate matter. The froth cleaning apparatus includes a hood including a lower peripheral edge for interface with the top of a froth flotation cell; a discharge orifice disposed in the hood; a froth support in communication with the discharge outlet for receiving and supporting froth; and a wash sprayer disposed upstream of the discharge orifice. The flotation apparatus includes: a wall defining a flotation cell; an aerator for aerating a mixture of particulate matter to produce froth; a feed opening for introducing a mixture of particulate matter and/or froth into the cell; a discharge orifice in a wall of the cell; a froth support in communication with the discharge outlet for receiving and supporting the froth; and a wash sprayer disposed upstream of the discharge orifice.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention generally relates to the concentration orbeneficiation of minerals and other particulate matter by frothflotation, and more particularly relates to a method and apparatus forconcentration or beneficiation of particulate matter separated fromundesired waste by means of froth flotation.

[0003] 2. Brief Description of Related Technology

[0004] Commercially valuable substances, such as coal and minerals, arecommonly found in nature mixed with relatively large quantities orprohibitive quantities of unwanted substances. As a consequence, it isusually necessary to beneficiate or clean ores to concentrate a desiredsubstance or, put another way, reduce the content of an unwantedsubstance. Similarly, recycling processes, such as de-inking of paperfibers, involve the separation of a desired substance (paper fibers)from an unwanted substance (ink).

[0005] Mixtures of finely-divided product particles and finely-dividedwaste particles can be separated and concentrates obtained therefrom byfroth flotation techniques. Generally, froth flotation involvesconditioning a liquid, commonly aqueous, pulp (or slurry) of the mixtureof product and waste particles with one or more frothing agents andoptional reagents, and aerating the pulp. The conditioned pulp isaerated by introducing into the pulp a plurality of air bubbles whichtend to become attached to either the product particles or the wasteparticles, thereby causing these particles to rise and generate a floatfraction of froth on the surface of a non-float fraction of pulp. Thedifference in density between air bubbles and water provides buoyancythat preferentially lifts hydrophobic solid particles to the surface. Inknown processes, the float fraction overflows or is skimmed from theflotation apparatus.

[0006] Froth flotation is often used to separate solids of similardensities and sizes, which factors prevent other types of separationsbased on gravity that might otherwise be employed. It is especiallyuseful for particle sizes below about 100 μm (about 150 mesh), which aretypically too small for gravity separation using jigging and tabling.The lower-size limit for flotation separation is typically about 35 μm(about 400 mesh). At smaller particle sizes, it becomes difficult totake advantage of surface-property differences to induce selectivehydrophobicity. On the other hand, particles greater than about 200 μm(about 65 mesh) tend to be readily sheared from bubble surfaces bycollision with other particles or vessel walls.

[0007] Today, at least 100 different minerals, including almost all ofthe world's copper, lead, zinc, nickel, silver, molybdenum, manganese,chromium, cobalt, tungsten, and titanium, are processed using frothflotation. Another major usage of froth flotation is by the coalindustry for desulfurization and the recovery of fine coal, oncediscarded as waste. Since the 1950's, flotation has also been applied inmany non-mineral industries including sewage treatment; waterpurification; paper de-inking; and chemical, plastics, and foodprocessing.

[0008] In conventional subaeration cells, the pulp ordinarily is aeratedby means of a mechanical impeller-type agitator and aerator whichextends down into the body of pulp and which disperses minute bubbles ofair throughout the body of pulp by vigorous mechanical agitation of thepulp.

[0009] In conventional froth-flotation columns, air for aeration isintroduced directly into a relatively quiescent body of pulp by means ofan air diffuser or sparger which is immersed in or in direct contactwith the pulp, or by introduction of pre-aerated water, e.g. from belowa flotation compartment.

[0010] Generally, subaeration cells have a relatively higher throughputthan froth-flotation columns, but froth-flotation columns can providebetter separation between desired and undesired components. As aconsequence, when both high throughput and good separation are desired,subaeration cells typically are used in series and froth-flotationcolumns are used in parallel. In some cases, the flotation operationsare conducted in stages wherein the concentrate obtained from the floatfraction in one stage can comprise a different substance from theconcentrate obtained from the float fraction in another stage.

[0011] Typical undesired impurities in coal include pyrite, sulfur, andother ash-forming mineral matter. Pyrite in many U.S. coals occurs inlarge quantities as fine-grained matter varying in size between 20microns (μm) and 32 μm. In some coals, such as is available in Illinois,a significant part of the pyrite is less than 20 μm. To make use ofthese types of coals more fully, a coal cleaning method capable ofprocessing very finely ground coal in which most of the pyrite particleshave been liberated must be used. Similarly, reduction in or removal ofash-forming matter can improve marketability and heat content of cleanedcoals, because ash is incombustible and has been linked to poor heatexchange and reduced boiler performance.

[0012] In addition, because every coal mine and preparation plantproduces fines in the course of extracting and processing coal, failureto recover coal from fines increases the proportion of produced coalthat is discharged into the environment (e.g., into tailing ponds) whichresults not only in a loss of potential revenue but also in anenvironmental impact.

[0013] The separation of fine particles by froth flotation techniquespresents particular obstacles which are only overcome with greatdifficulty and cost by known techniques, such as use of multiplemachines in series or parallel, and known techniques still havelimitations in the degree of separation which can be achieved.

[0014] Thus, it is a continuous goal in the industry to have methods andapparatus which improve the separation of desired particulate matterfrom undesired particle matter.

SUMMARY

[0015] One aspect of the disclosure is a method of separating a desiredconstituent from a mixture of particulate matter, including the steps ofaerating a pulp to generate a float fraction of froth supported on thesurface of a non-float fraction of pulp, separating a portion of frothfrom the float fraction, draining the separated froth, washing theseparated froth to dislodge one or more undesired constituents, andrecovering the washed froth.

[0016] Another aspect of the disclosure is an apparatus for separating adesired constituent from a mixture of particulate matter, the apparatusincluding at least one wall, preferably walls, defining a flotationcell; an aerator for aerating a mixture of particulate matter to producefroth; a feed opening for introducing a mixture of particulate matterand/or froth into the cell; a discharge orifice in a wall of the cell; afroth support in communication with the discharge orifice for receivingfroth from the cell and supporting the froth; and a wash sprayerdisposed upstream of the discharge orifice. If the apparatus is operatedin a semi-batch or continuous mode, then preferably the apparatus willinclude a liquid drain in a wall of the cell, for example in the lowerend of the cell.

[0017] Still another aspect of the disclosure is a froth cleaningapparatus for use with a froth flotation cell that causes froth tocollect at the top of the cell. The apparatus will generally include allof the elements of the apparatus described above, except that variouselements (such as the walls of the cell, the aerator, and other elementsfound in flotation cells) can be provided with the flotation cell. Thus,one embodiment of such a cleaning apparatus includes a hood including alower peripheral edge for interface with the top of a froth flotationcell; a discharge orifice disposed in the hood; a froth support incommunication with the discharge orifice for receiving pulp and/or frothand supporting the froth; and a wash sprayer disposed upstream of thedischarge orifice.

[0018] Further aspects and advantages of the invention may becomeapparent to those skilled in the art from a review of the followingdetailed description, taken in conjunction with the appended claims.While the invention is susceptible of embodiments in various forms,described hereinafter are specific embodiments of the invention with theunderstanding that the disclosure is illustrative, and is not intendedto limit the invention to the specific embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a partial cross-sectional view of a froth flotationapparatus according to the disclosure.

[0020]FIG. 2 is a partial cross-sectional view of the froth cleaningsection of the apparatus of FIG. 1.

[0021]FIGS. 3 and 4 show partial cross-sectional views of a frothcleaning section of an apparatus according to the disclosure.

[0022]FIGS. 5 and 6 show partial cross-sectional views of a variation ofa froth cleaning section of an apparatus according to the disclosure.

[0023]FIGS. 7 and 8 show partial cross-sectional views of a variation ofa froth cleaning section of an apparatus according to the disclosure.

[0024]FIG. 9 is a partial cross-sectional view that shows the interfaceof an embodiment of a froth cleaning apparatus according to thedisclosure with a type of flotation cell having an overflow weir andlaunder around the circumference of the cell, such as those known in theart.

[0025]FIG. 10 is a partial cross-sectional view of a froth cleaningapparatus according to the disclosure including a froth support having amovable upper section to control the depth of froth passing through thecleaner.

[0026]FIG. 11 shows a partial cutaway view of a froth support apparatusthat includes a repositionable panel to change the height of a passageformed therewith from the bottom to control the depth of froth passedthrough the passage.

[0027]FIG. 12 is a partial cross-sectional view of a froth flotationapparatus according to the disclosure that includes a froth cleaningsection around the circumference of the major body of a cell and a hoodto allow a larger volume for froth cleaning.

[0028]FIGS. 13 and 14 show partial cross-sectional views of a variationof a froth cleaning section of an apparatus according to the disclosure.

[0029]FIG. 15 shows a cross-section of a froth flotation apparatusaccording to the disclosure that includes a generally zigzag-shapedfroth cleaning section.

[0030]FIG. 16 shows a froth cleaning apparatus according to thedisclosure interfaced with a laboratory-scale flotation column.

[0031]FIG. 17 shows a cross-sectional view of the cleaning section ofthe apparatus in FIG. 16, showing a grooved lower support surface.

[0032]FIGS. 18 and 19 are charts comparing ash and sulfur rejectioncapabilities, respectively, of a conventional flotation column withfroth washing and a conventional flotation column equipped with a frothcleaning apparatus according to the disclosure. The curves shown in thetwo figures plot the best possible cleaning performance according toAdvanced Flotation Release analysis, as described below.

[0033]FIGS. 20 and 21 are charts comparing sulfur and ash rejectioncapabilities, respectively, the same apparatus used to generate FIGS. 18and 19.

[0034]FIG. 22 shows the results of carrying capacity tests for anapparatus according to the disclosure.

[0035]FIG. 23 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids) of Illinois fine coal (8%ash) in AFR analysis and with a subaeration cell modified according tothe disclosure.

[0036]FIG. 24 plots combustible recovery versus total sulfur content ofthe product separated from a slurry (5% solids) of Illinois fine coal(3.17% sulfur) in AFR analysis and with a subaeration cell modifiedaccording to the disclosure.

[0037]FIG. 25 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids) of a reconstituted feed (24%ash) in AFR tests and with a subaeration cell modified according to thedisclosure.

[0038]FIG. 26 plots combustible recovery versus total sulfur content ofthe product separated from a slurry (5% solids) of a reconstituted feed(3.44% sulfur) in AFR tests and with a subaeration cell modifiedaccording to the disclosure.

[0039]FIG. 27 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids) of high-ash Illinois coalrefuse (40% ash) in AFR tests and with a subaeration cell modifiedaccording to the disclosure.

[0040]FIG. 28 plots combustible recovery versus total sulfur content ofthe product separated from a slurry (5% solids) of Illinois coal refuse(3.7% sulfur) in AFR tests and with a subaeration cell modifiedaccording to the disclosure.

[0041]FIG. 29 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids; 24% ash) of fine coal mixedwith washing plant rejects (tailings) in AFR tests, with a subaerationcell modified according to the disclosure, and with a 1.3 ft³subaeration cell modified according to the disclosure to process a pulp(3% solids) containing fine coal with 26% ash drawn from the rejectstream of an Illinois coal preparation plant.

DETAILED DESCRIPTION OF EMBODIMENTS

[0042] The invention generally relates to a method and apparatus forseparation and concentration of particulate matter, and is particularlyadvantageous for beneficiation of coal fines.

[0043] One aspect of the disclosure is a method of separating a desiredconstituent from a mixture of particulate matter, including the steps ofaerating a pulp to generate a float fraction of froth supported on thesurface of a non-float fraction of pulp, separating a portion of frothfrom the float fraction, draining the separated froth, washing theseparated froth to dislodge undesired constituents, and recovering thewashed froth.

[0044] The mixture of particulate matter is combined with a liquid and afrothing agent to create a pulp. The frothing agent stabilizes bubblesthat are created by the introduction of air into the pulp. For practicalreasons, the major liquid component of the pulp typically will be water,though this need not be the case.

[0045] The mixture of particulate matter is not limited to any specifictype of compounds. Typical applications can include mineral processingand waste processing. Examples include separation of coal from mixturesincluding gangue materials such as sulfur, pyrite, and other ash-formingmaterials; separation of cellulosic (e.g., paper) fibers from mixtureswith ink; separation of dolomite from phosphate-containing minerals;separation of hematite from quartz; separation of chalcopyrite and/orchalcocite from silicate gangue minerals; separation of zeolite fromquartz; separation of quartz from magnesite; separation of calcite fromapatite; separation of feldspar from nepheline syenite; and separationof silica-containing minerals from limestone.

[0046] The mixture of particulate matter is not limited to any specificparticle sizes; however, the method and apparatus disclosed herein offersignificant advantages over known methods of processing mixtures withvery fine particle sizes, such as less than about 0.65 mm.

[0047] A frother (frothing agent) can be added to promote the formationof stable bubbles under aeration. A frother preferably includes both apolar end and a nonpolar end. The type and amount of frothing agentsuitable will vary with the mixture of particulate matter and theconstituent desired to be separated, similar to known art. Frothers aregenerally classified by their polar groups, with the most common beinghydroxyl, carboxyl, carbonyl, amino, and sulfo groups. Preferably, thefrother will contain at least five or six carbon atoms in astraight-chain, nonpolar group for sufficient and stable interactionwith the gaseous (e.g., air) phase. For branched-chain hydrocarbons, thenumber of carbon atoms in the nonpolar group preferably is about 16 orless. For example, methyl isobutyl carbinol (MIBC) is a suitablefrothing agent for use with coal. Often, a suitable type and amount offrothing agent cannot be accurately predicted for a particular type ofore, but can quickly be determined by empirical testing. The type offrothing agent is typically a secondary consideration, chosen after acollector (described below), to provide suitable frothing conditionswithout interfering with a collector or separation system (i.e.,combination of a collector and other optional reagents).

[0048] Optionally, the pulp can contain one or more reagents selectedfrom collectors, activators, depressants, dispersants, and othermodifiers (e.g., pH modifiers). Collectors are used to invoke selectivehydrophobicity, and are typically heteropolar organic substances. Thenonpolar end is almost always a long-chain or cyclic hydrocarbon groupthat is hydrophobic. Preferably, a suitable collector will selectivelyinduce hydrophobicity on the desired material to be recovered whileretaining hydrophilicity of the nondesirable material. Both anionic andcationic collectors can be used. Examples of anionic collectors includesodium oleate, xanthates, dithiophosphates, alkyl sulfuric salts. Themost common cationic collectors include amine groups, such as anilineand pyridine, and are optionally used with an acid to induce solubility.Because of the inherent heteropolarity of collectors, a collector canalso serve as a frother in some systems (e.g., sodium oleate andsulfo-soliophil fatty acids).

[0049] The natural hydrophobicity of coal (especially freshly groundcoal) is an asset which minimizes the use of collectors. However, onextended storage, or if the coal or coal seam has been in contact withair for a few years (even days, in some circumstances), the coal cantend to lose much of its hydrophobicity, and is often then referred toas “weathered coal.” The addition of a collector, such as hydrocarbonoils (e.g., kerosene or fuel oil), can make coal more floatable.Typically-used alcohols include MIBC, and amyl, hexyl, heptyl and octylalcohols. Typical amounts of oils and alcohols per 1 ton of coal areabout 50 gr/ton to about 250 gr/ton and about 250 gr/ton to about 1000gr/ton, respectively.

[0050] Activators can be added to chemically “resurface” the solid toincrease interaction with collectors that are otherwise ineffectivealone. Depressants form a polar chemical envelope around a solidparticle that enhances hydrophilicity or selectively preventsinteraction with collectors that might induce unwanted hydrophobicity.Dispersants act to break agglomerated particles apart so that singleparticles interact with the collector and air bubbles. A pH modifier(e.g., an alkaline modifier such as caustic soda, lime, and soda ash;and an acidic regulator such as hydrochloric and sulfuric acids) can beused, because the hydrophobicity of systems is often optimal within aparticular pH range, and some frothers require a certain pH range toform stable bubbles. A pH modifier can, in some cases, also serve as oneor more of a dispersant, a depressant, and an activator.

[0051] The pulp is aerated to generate a float fraction of frothsupported on the surface of a non-float fraction of pulp. The floatfraction includes a plurality of bubbles, at least a portion of whichare selectively attached to a desired constituent of the pulp. Aerationcan be achieved by various techniques, including a sparger, impeller,agitator, and the like, disposed within the pulp. As used herein, theterm “aerate” and forms thereof are defined to relate to not only airbut also any other gaseous substance. For example, nitrogen bubbles havebeen used to effect the separation of copper sulfide minerals frommolybdenum sulfide minerals. In a preferred embodiment, aeration isachieved via the introduction of air through the shaft of an agitator,and exits into the pulp at the bottom of the agitator (i.e.,subaeration). In another embodiment, aeration can be achieved via anaerator that includes an injector jet disposed in a conduit and a sourceof air in communication with the conduit, such as disclosed in U.S. Pat.No. 4,938,865 (Jul. 3, 1990), the disclosure of which is incorporatedherein by reference. By use of such an aerator, air can be entrainedinto liquid pulp, for example to create a downward-moving foam bed in acolumn that enters from the lower part of the column into a vessel, inwhich the froth separates from liquid, forming a liquid(pulp)/frothinterface.

[0052] By use of a method and apparatus according to the disclosure, theamount of air required by the system is surprisingly substantiallyreduced compared to known methods. For example, a 100 ft³ (about 2.8 m³)subaeration cell with an open top would typically require about 300ft³/min (about 8.5 m³/min) of air supply (e.g., from a blower), whereasa similarly-sized cell according to the disclosure herein that includesa closed top would require only about 15 ft³/min (about 0.4 m³/min) ofair supply.

[0053] The location (typically, the level within a vessel) of theliquid/froth interface is a variable that can be adjusted by theoperator of a flotation cell. In a preferred method according to thedisclosure wherein very fine froth is produced, the liquid/frothinterface is formed near the nominal “top” of the primary enclosure ofthe vessel, more preferably at or above the top. Thus, in oneembodiment, the liquid/froth interface is formed at a position slightlyabove the top of the primary enclosure of the vessel, such that the areaof mixing within the apparatus is further separated, preferablyisolated, from the area of particle separation. Very fine froth can beproduced by various methods, such as by using a high concentration offlotation reagent, or by using an emulsified flotation reagent, such asEKOFOL 440 kch reagent available from EKOF Flotation GmbH of Bochum,Germany. This aspect of the preferred methods is described below inconnection with embodiments of the apparatus disclosed herein.

[0054] A portion of the froth is substantially separated from the floatfraction of froth. Thus, in one embodiment of the method, froth risingfrom the liquid/froth interface continuously pushes already-formed froth(in layers above) up the slope of an upwardly-inclined surface of afroth washer, such that the already-formed froth is then supported, atleast in part, by the upwardly-inclined surface of the froth washer. Theproduct-laden bubbles escape from the surface of the slurry and move upthe surface of the support. Without such a separation, thealready-formed froth would continue to rise, and as more froth isgenerated below the weight above would tend to reach a critical valueand destabilize a portion of the froth layer. In another embodiment,froth from the float fraction can be separated in other ways, includingmechanical methods, such as by pickup and conveyance along a moving beltdisposed, for example, at, adjacent to, or above the liquid/frothinterface. As another example, froth can be separated via other pressuredifferentials (e.g., discharge by force from produced froth, pneumaticconveyance such as from higher air pressure in the cell, suction, andthe like). Other ways of achieving separation will be apparent to thoseof skill in the art in view of the disclosure, and the disclosure is notmeant to be limiting.

[0055] Furthermore, separation can facilitate improved washing of thefroth. In known processes wherein froth supported on a layer of pulp iswashed, the non-selectively attached particles travel downwardly (i.e.,in the direction of gravity) through layers of froth below, increasingthe probability that the particles washed from the top layer will attachto any one of the layers of bubbles below. While in theory the flow offroth in such processes over a simple weir according to the prior artwill naturally tend to include froth from the top layers of the floatfraction, this is not always the case, and froth containing reattachedparticles also overflows the weir. In addition, even particles that arenot attached to froth tend to be carried over the weir with overflowingfroth. In contrast, by practice of a method described herein wherein aportion of froth from the float fraction is separated, the opportunitiesfor washed particles to reattach to discharged froth are very muchreduced, and the yield of desired constituents of the pulp issurprisingly increased.

[0056] Moreover, separation of the froth from the float fraction, canoptionally allow for drainage of the froth at various phases of theprocess. As the froth is drained of liquid, nonselectively-attachedparticles and particles that might otherwise be carried over into theproduct are removed (e.g., entrapped and entrained particles). Thus, forexample, as a portion of froth is pushed into and up the froth washingchute described below, the froth is substantially drained of pulp liquidand particles (e.g., nonselectively-attached particles) beginning at thetime of separation by such liquid running down the surface of theupwardly-inclined froth support (under the froth) and back into thecell. Thus, a froth drainer can include an upwardly-inclined frothsupport, a froth support including a perforated region for moreefficient separation of wash liquid (e.g., porous or containing otherholes, such as a screen), an entirely perforated support, and the like.

[0057] Preferably, the froth is at least partially drained beforewashing, preferably substantially drained. In addition, if the froth issubsequently washed in such an apparatus, then the froth preferably willbe drained of wash water and particles both during and subsequent towashing.

[0058] A method of separating or concentrating a desired constituentfrom a mixture of particles as described herein includes a step ofwashing separated froth with a liquid to dislodge undesired particlesfrom the froth. The undesired particles typically will includenon-selectively attached, entrained, and entrapped particles, andmixtures thereof. The wash liquid preferably includes as a majorcomponent a liquid that is also used in the pulp, for convenience andcompatibility. Thus, when the pulp includes water, then preferably thewash liquid is water. The wash liquid can also include one or moreadditional agents, such as a frother, collector, activator, depressant,dispersant, or other modifier. Preferably, the washing step includesmore than one washing operation, and more preferably includes aplurality of washing operations in series, such that each portion ofseparated froth is washed more than once.

[0059] The better the separation of the wash liquid from the separatedfroth, the lower is the risk that dislodged particles in the wash liquidwill reattach to bubbles in the separated froth as the wash liquid isdrained from the froth. Thus, the wash liquid that includes dislodgedparticles preferably is substantially separated from contact with theseparated froth, and more preferably is also separated from the washedfroth. This can be achieved for example, via an apparatus describedbelow wherein the separated froth travels up an upwardly-inclinedsurface, such that the wash water flows down the upwardly-inclinedsurface and has the opportunity to contact only the lowermost layer ofbubbles in the separated froth. Preferably, such an upwardly-inclinedsupport can be provided with a nonplanar (e.g., toothed or crenate,crenellated, corrugated, and the like) surface profile, such that thewash liquid collects and travels at the lower extremities of theprofile, further separating the froth from the wash liquid. Other waysof achieving such separation will be apparent to those of skill in theart in view of the disclosure, and the disclosure is not meant to belimiting.

[0060] As described above, the depth of a layer of froth can affect thestability of the bubbles at the bottom of the froth, and can also affectthe effectiveness of washing the froth. For example, the deeper a massof froth (more layers), the more likely it is thatnonselectively-attached particles washed from a top layer will reattachor become entrapped in a lower layer of froth. On the other hand, as thedepth of froth decreases, the throughput of the process tends todecrease In addition, the ability to reject fine contaminant particlesappears to decrease with increasing depth, especially for fine particlessuch as clay and pyrite that have relatively slow settling velocities.Thus, the depth of the separated froth preferably is controlled toachieve a desired balance between factors including those describedabove. The depth of froth can be controlled, for example, by controlling(e.g., fixed or variable) the height of an opening through which frothis discharged from a flotation chamber or, in addition, by forcing froththrough a conduit having a controlled (fixed or variable) height. Inpreviously-known froth flotation apparatus, the froth depth must be ofsufficient height to permit froth to flow over a weir withoutsignificant carryover of pulp. In contrast, in a method and apparatus asdisclosed herein, the froth depth can be relatively smaller, because aportion of the float fraction is separated from the pulp and can bewashed and drained while separated. Accordingly, provision of a frothsupport allows the effective height of a froth column to be increasedwithout affecting the stability of the lower layers of the froth,because much of the mass of the froth is supported by the froth supportrather than the underlying bubbles.

[0061] The washed froth can be recovered by collecting the froth in alaunder as it is discharged at the outlet of a froth washing apparatusor via flow over a weir, with or without mechanical skimming, forexample.

[0062] The hydrophilic particles remain in the pulp solution, which iseventually decanted off as waste (e.g., a tailing stream), in either abatch, semi-batch, or, preferably, a continuous process. Typically theconcentrated stream contains at least one desired, or more valuable,component and the tailing stream contains one or more less valuablecomponents (e.g., gangue). In some applications, however, the morevaluable component will be in the tailing stream and a less valuablecomponent will be desired to be removed in the concentrated stream fromthe froth.

[0063] Another aspect of the disclosure is an apparatus for separating adesired constituent from a mixture of particulate matter, the apparatusincluding at least one wall (e.g., a sphere), preferably walls, defininga flotation cell (e.g., a cylindrical vessel); an aerator for aerating amixture of particulate matter to produce froth; a feed opening forintroducing a mixture of particulate matter and/or froth into the cell;a discharge orifice in a wall of the cell; a froth support incommunication with the discharge orifice for receiving froth from thecell and supporting the froth; and one or more wash sprayers disposedupstream of the discharge orifice. If the apparatus is operated in asemi-batch or continuous mode, then preferably the apparatus willinclude a liquid drain in a wall of the cell, for example in the lowerend of the cell. At least a portion, preferably the majority, of thefroth support is disposed at or above the top of the flotation cell. Inoperation, at least a portion, preferably the majority, of the frothsupport is disposed at or above the froth/liquid interface.

[0064] Still another aspect of the disclosure is a froth cleaningapparatus for use with a froth flotation cell that causes froth tocollect and/or be confined at the top of the cell. The apparatus willgenerally include all of the elements and aspects of the apparatusdescribed above, except that various elements (such as the walls of thecell, the aerator, and other elements found in known flotation cells)can be provided with the flotation cell. Thus, one embodiment of such acleaning apparatus includes a hood including a lower peripheral edge forinterface with the top of a froth flotation cell; a discharge orificedisposed in the hood; a froth support in communication with thedischarge orifice for receiving froth (and, optionally, pulp) andsupporting the froth; and one or more wash sprayers disposed upstream ofthe discharge orifice.

[0065] Either of the above-described apparatus will preferably includeone or more features selected from agitators, froth depth controllers,froth motivators (mechanical or otherwise), mesh screens, and perforatedplates (for example, disposed in the path of travel of at least aportion of the generated froth).

[0066] In a froth flotation apparatus according to the disclosure, thewall or walls defining the flotation cell preferably will define asubstantially cylindrical vessel (column) that, in contrast to knownflotation cells, is at least substantially closed at the top. Thecylinder can have a circular, elliptical, square, rectangular, or anyother shape of cross section. The length (height) of the column willtypically be greater than the width.

[0067] The aerator serves to introduce a gas into a mixture ofparticulate matter to produce bubbles. The aerator can be selected, forexample, from a diffuser, sparger, agitator, and impeller disposed in orin direct contact with the pulp. In a preferred embodiment, aeration isachieved via the introduction of air through the shaft of an agitator,and exits into the pulp at the bottom of the agitator (e.g., asubaeration cell). The aerator can also be located outside a body ofpulp, for example a diffuser or sparger that introduces pre-aeratedliquid (e.g., water) from outside the flotation compartment. In anotherembodiment, the aerator located outside a body of pulp can include aninjector jet disposed in a conduit and a source of air in communicationwith the conduit, such as disclosed in U.S. Pat. No. 4,938,865,described above.

[0068] A froth flotation apparatus includes a discharge orifice in awall of the cell. Similarly, a froth cleaning apparatus includes adischarge orifice disposed in the hood. The orifice provides a path forfroth and/or pulp to exit the cell. When the top of the orifice extendsabove the froth support to form a passage of controlled height (e.g., toform a flap or hood and the like over a froth support (e.g., adiscontinuous cross-sectional perimeter) or to form a conduit, channel,chute, or pipe, and the like (e.g., a continuous cross-sectionalperimeter ) with a froth support), it can also serve as a froth depthcontroller, for example if froth is generated within the cell and exitsthrough the orifice at an angle (i.e., not vertically).

[0069] In either the froth flotation apparatus or the froth cleaningapparatus, a froth support is in communication with the dischargeorifice for receiving and supporting froth. The froth support can takeany form suitable for conveying and, preferably, draining froth of thedesired depth. The froth support can be a single member or an apparatusincluding a plurality of individual members. Suitable examples include aclosed cylindrical conduit and a closed conduit having a rectangularcross-section (e.g., a chute). Preferably, the froth support includes abottom interior surface (e.g., a major surface supporting the froth)that has an average width greater than the average height of thesupport. Thus, the width of the conduit will be greater than the heightof the conduit, for example.

[0070] At least a portion, preferably the majority, of the froth supportis disposed at or above the top of the flotation cell. In operation, atleast a portion, preferably the majority, of the froth support isdisposed at or above the froth/liquid interface. Accordingly, suchembodiments will facilitate drainage of the froth, as opposed to frothsimply flowing over a weir as in the prior art.

[0071] In one embodiment, the froth support includes a froth supportsurface that has a non-planar profile. As described above, such afeature can serve to promote drainage and separation of froth fromdrained pulp and/or wash water. Examples include crenate, crenellated,and corrugated profiles. The profile can also be substantially smoothand concave (to direct drained liquid to the center) or convex (todirect drained liquid to the edges).

[0072] In another embodiment, the froth support includes a froth supportsurface that is perforated (e.g., a perforated or porous plate or ascreen) to promote drainage of froth and removal of liquid while stillsupporting the froth, or at least a majority of the bubbles of thefroth. Such a perforated froth support can include a channel, conduit,or the like for collecting drained liquid for return to the flotationcell or recycle. For example, the collected drained liquid can berecycled to make up additional pulp, or can be filtered to provide arecycle wash water stream. Alternatively, the collected drained liquidcan be discharged from the process.

[0073] When the froth support has a continuous cross-sectional perimeter(e.g., to form a substantially closed channel, chute, conduit, or thelike), such that gas (e.g., air) introduced into the cell acts as adriving force, especially when it is the sole driving force, for frothto move through the support then preferably the support includes a ventfor release of air. The vent preferably is located at the top of thesupport. The distance of the vent from the discharge opening of thecell, and the size of the vent, will tend to affect pulp and froth flowdynamics. For example, for a pulp in which a relatively high frequencyof collision between bubbles and pulp particles is desired, then a highflow rate of air will be desired in the cell, and a relatively largevent and/or a vent located closer to the main body of the cell will bepreferred. For an application wherein a relatively long froth support isdesired to facilitate cleaning of the froth, then a vent, when used,will preferably be located relatively far from the discharge from thecell, such that the froth has sufficient motivation force behind it topropel it through the support. Accordingly, in a preferred embodiment,one or more of the vent size and the vent location is adjustable. Thevent may be a simple hole or can advantageously include a conduit (e.g.,a stack or chimney) to prevent substantial loss of froth through thevent. Such a stack can include a damper to permit control over thevolume of gas flow.

[0074] When the froth support includes a conduit, the conduit caninclude one or more features to control the depth of froth. For example,the conduit can have a tapered cross section, wherein the end of theconduit in proximity to the first discharge outlet in the wall of theflotation cell or in the hood can have a relatively larger cross-sectionthan the opposite end of the conduit. Such a feature can serve toincrease the depth of the froth in the case where a portion of thebubbles in the froth are destabilized during washing. A tapered conduitcan alternatively have the reverse configuration, wherein the end of theconduit in proximity to the first discharge outlet in the wall of theflotation cell or in the hood has a relatively smaller cross-sectionthan the opposite end of the conduit. In such an embodiment, the depthof froth will decrease as the froth progresses through the conduit, suchthat the froth depth is at a minimum during its final washing, wherebythe opportunity for reattachment or entrapment of dislodged particles isminimized and thereby a concentrate of high purity is obtained.

[0075] The froth support can also include one or more movable parts tocontrol the depth of froth. For example, in an embodiment describedbelow, an upper panel of a froth chute can be selectively positionedrelative to the lower panel of the froth chute, to provide a variableheight in the chute. Similarly, the bottom panel of such a chute canalso be adapted for variability.

[0076] The froth support preferably is disposed at an upwardly-inclinedangle, such that froth is conveyed up the support. For example, if theflotation cell includes a base having a major plane, the froth supportpreferably is disposed at an upwardly-inclined angle with respect to themajor plane of the base. As for the froth cleaner apparatus adapted foruse with a flotation cell, when the lower peripheral edge of the hoodsubstantially lies in a plane, the froth support can be disposed at anupwardly-inclined angle with respect to that plane. Put another way, thefroth support preferably is disposed at an angle greater than 90 degreeswith respect to the gravity vector when the apparatus is in operation.In similar fashion, when either of the apparatus is in operation, thefroth support preferably is disposed at an upwardly-inclined angle withrespect to the liquid/froth interface. For example, when concentratingcoal fines in an apparatus wherein the froth height is controlled toabout 1 inch (about 2.54 cm), then preferably the support angle is about25 degrees, e.g. 22.5 degrees.

[0077] As the froth height is increased, then the angle preferably isalso increased. As a result of inclination, the distance that the frothmust traverse before being removed from the apparatus is generallygreater for a given total froth depth than for a traditional verticalcolumn of froth of the same depth, but the static pressure on the lowerlayers of the froth is less than a vertical column of froth of the sameheight. When operating conditions in the cell present the possibilitythat too much contaminant material will be carried over with theconcentrate, a higher angle of inclination can be used. This could bethe case, for example, when using a more coarse material and/or when theslurry level is raised and the column of froth within the cell (asopposed to the cleaner section) is kept to a minimum. Under someconditions, the slurry level can actually rise into the lower part ofthe upwardly-inclined froth cleaner with the froth bearing the productparticles emerging from the slurry within the froth cleaning device.Stable froth conditions in the cell may also allow the operator to lowerthe slurry level, maintain a thicker froth column in the cell, and lowerthe angle of upward inclination of the washer. Adjustment of the angleof upward inclination of the washer gives the operator another option tocontrol the performance of the cell for given feed materials, rates ofthroughput, and other variables.

[0078] One class of froth support embodiments designed to cope withpotential intermittent surges of pulp and concomitant or resultingchanges in the position or level of the liquid froth interface includesa non-linear pathway for fluid travel. Thus, for example, when aflotation cell includes a base having a major plane, then the frothsupport includes first and second sections disposed at upwardly-inclinedangles (same or different) with respect to the major plane of the baseand the first and second sections are disposed in opposing directionswith respect to a plane perpendicular to the major plane of the base. Asfor the froth cleaner apparatus adapted for use with a flotation cell,when the lower peripheral edge of the hood substantially lies in aplane, the froth support includes first and second sections disposed atupwardly-inclined angles (same or different) with respect to the lowerperipheral edge and the first and second sections are disposed inopposing directions with respect to a plane perpendicular to the lowerperipheral edge.

[0079] Preferably, the support has a convoluted path for fluid travel(e.g., tortuous, serpentine, winding, or boustrophedonic). If the pulplevel intermittently rises (e.g., in surging fashion), then one or moredeviations in fluid path flow can aid in preventing the surging pulpfrom contaminating cleaned froth, for example by spilling over a weirinto a launder. Such a support preferably includes one or more openingsand/or vents for discharge of air. A support having a convoluted pathfor fluid travel thus can provide a path for easy escape of excess airor gas introduced into a flotation cell without causing excessivespillage from froth from the washer before it can be adequately cleaned.

[0080] In an alternative arrangement, the support can be constructed insuch a way that its overall length may be increased or reduced (e.g.,via a telescopic mechanism) in which one part of the support slidesinside another, or by the addition or subtraction of segments of support(e.g., with the same cross-sectional area at segment interfaces), and ofa convenient incremental length.

[0081] In either the froth flotation apparatus or the froth cleaningapparatus, one or more wash sprayers (e.g., nozzles) are disposedupstream of the discharge orifice. A wash sprayer can be disposed abovethe surface of the froth support adapted to support froth, preferably inthe upper wall and/or side walls of a froth chute. The wash sprayerserves to dispense wash liquid onto and/or into the froth as it isconveyed along the froth support, for example in a cone or planeconfiguration. Preferably, the wash sprayer will provide a screen ofwash liquid through which the froth passes. Thus, preferably, a sprayeris disposed tangential to the flow of froth, and above the frothsupport. When more than one wash sprayer is used, preferably thesprayers are disposed in series, such that each portion of froth iswashed more than once. When each sprayer does not provide wash liquid tothe entire height and/or width of froth, the wash sprayers can bedisposed in rows or in staggered relationship such that the entireheight and/or width of froth undergoes at least one application of washliquid. In an embodiment such as that described in the Examples below,the turbulence of wash spraying can be increased to a point that woulddestroy the froth in a typical vertical flotation column; however, dueto the very wet and flowing conditions present in the cleaner section,the froth simply becomes more fluid and clean.

[0082] The froth support preferably includes a weir over which thewashed froth can flow into a launder for recovery.

[0083] Either the froth flotation apparatus or the froth cleaningapparatus can also include one or more screens (e.g., a mesh screen) orperforated plates through which a portion of generated froth can travel.For example, the apparatus can include a screen disposed in the path oftravel of separated froth in proximity to and/or upstream of thedischarge orifice. A screen can serve to separate bubbles to facilitaterelease of undesired particles entrapped between bubbles, and can helpto deter the flow of liquid pulp (e.g., surging pulp), for example toprevent liquid pulp from flowing out through the froth washing deviceand contaminating washed product. In one embodiment, the froth supportincludes two plastic plates perforated with 0.25-inch (6.4 mm) diameterholes, the plates disposed a short distance (e.g., about an inch) apart.The holes in the plates had offset centers, which forced the froththrough a tortuous path while partially blocking surging pulp fromtravelling high up into the washing section of the support.

[0084] Either of the apparatus can also include one or more frothmotivators, including a mechanical froth motivator such as thosedescribed below in connection with the figures, to assist in conveyanceof froth upstream of the discharge orifice.

[0085] A froth flotation apparatus equipped with a froth cleaning systemas disclosed herein can be used alone and can also be used in series,for example whereby the cleaned product and/or the tail stream can befed to a subsequent flotation apparatus.

[0086] Specific embodiments are described below in connection with thefigures.

[0087]FIG. 1 depicts a froth flotation apparatus that includes a wall 12defining a flotation cell 10 containing a pulp 14 that enters through afeed opening 18 via a feed conduit 20. An agitator 22 driven by a motor24 is disposed within the interior of the flotation cell 10. Air 28 isfed through the shaft 30 of the agitator 22 for release into the pulp 14at the bottom of the agitator 22 to achieve subaeration. Pulp containinga relatively higher concentration of undesired particles is dischargedfrom the cell through orifice 32 and conduit 34 at the bottom of thecell 10.

[0088] The apparatus also includes a froth cleaner section 38 includinga froth support 40 disposed at an angle a with respect to the majorplane of the base 42 of the flotation cell 10. Pulp and/or froth entersthe cleaner section 38 via orifice 44. The support 40 is in the form ofa rectangular chute, with an overflow weir 48 leading to launder 50.Wash sprayers 52 are disposed in a top panel 46 of the chute in seriesto spray wash liquid on froth 54. Air 28 introduced through the agitator22 exits through opening 56. The apparatus in FIG. 1 is shown in amanner of operation wherein the liquid/froth interface 58 is at the topof the cell 10 and, thus, within the cleaner section 38. Operation inthis manner is preferred to achieve high throughput and vigorousagitation in the interior of the cell 10.

[0089]FIG. 2 is a cross-sectional view of the froth cleaning section 38at the location indicated on FIG. 1, wherein like reference numbersindicate like elements. FIG. 2 shows a sprayer 52 disposed above acrenate (toothed) bottom panel 60 of the froth support 40. In thisembodiment, wash liquid will preferentially flow in areas 60 a of thefroth support, whereas the froth will tend to be supported above.

[0090] Another advantage of this embodiment is that it includes no moremoving parts than a traditional flotation column which washes the frothat the top of the column and, particularly with respect to a subaerationcell, no additional moving parts in direct contact with the pulp, whichcan be rather dense (e.g., up to 60% or more solids) and abrasive. As aconsequence, operating costs, and in particular the cost of maintenanceand repairs, are no greater than for a flotation column or subaerationcell without a froth cleaning device disclosed herein, whereas betterseparation is obtained.

[0091]FIGS. 3 and 4 show a partial view and a cross-sectional view,respectively, of a froth cleaning section 70 of a froth flotationapparatus according to the disclosure. This apparatus includes aplurality of mechanical froth motivators in the form of froth pushers 72attached to a belt 74 supported by spindles 78, one or more of which aredriven to rotate the belt. As a consequence of including frothmotivators, this apparatus can be disposed at a lower upwardly-inclinedangle. In the apparatus shown, wash sprayers 82 are disposed in sidewalls 84 and 86 (FIG. 4) of the froth support 90. The froth support 90is shown with a crenate bottom panel 92, and the pushers 72 are disposedsuch that an edge of each pusher 72 comes in proximity to or in contactwith the panel 92 at upper zones 92 b to move the majority of froth 94through the channel formed by the belt 74 and the support 90 in thelower region of the support 90. As in the embodiment shown in FIG. 3,wash liquid will preferentially flow in areas 92 a of the froth support.The support 90 includes an air exit opening 88 and an overflow weir 96leading to launder 98. The embodiment as shown inludes a top panel 76,though this need not be the case as long as froth is prevented fromescaping the apparatus via other means, such as by provision of asealing flap (not shown) which can be disposed at region 80, forexample, to interface with the belt 74 but still allow passage of themotivators 72.

[0092]FIGS. 5 and 6 show another variation of an apparatus including afroth cleaning section 99, FIG. 5 being a partial view and FIG. 6 beinga corresponding cross-sectional view. This apparatus also includes aplurality of mechanical froth motivators in the form of froth pushers100 mounted on a belt 102 on spindles 104 and including a series of washsprayers 108. In this embodiment, the belt 102 and pushers 100 move inthe clockwise direction, as shown, such that the froth is moved throughthe channel formed by the belt 102 and the support 110 in the upperregion of the support 110, and the sprayers 108 are disposed in a top112 of the support 110 to wash the froth that passes by.

[0093] The pushers 100 in this embodiment preferably intersect with thesupport 110 at the bottom panel 114 in a substantially sealing fashion,such that only insubstantial amounts of pulp and/or froth are allowed toenter the bottom channel formed by the belt 102, the bottom wall 1 14,and side walls 118 and 120 of the support 110 in a direction counter tothe belt movement. For example, pushers 100 can be constructed of aflexible material and in a length slightly greater than the bottomchannel, such that the pushers 100 bend when traveling through thebottom channel. In the embodiment shown, the belt 102 is shown in FIG. 4to have grooves 122 (shown with a triangular cross-section) throughwhich wash water can flow back into a flotation cell (not shown). Theweir 124 in such an embodiment is positioned in such a manner that atleast a majority of the washed froth 126 flows into the launder 128 andair exits through opening 130.

[0094]FIGS. 7 and 8 show another variation of an apparatus including afroth cleaning section 132, FIG. 7 being a partial view and FIG. 8 beinga corresponding partial cross-sectional view. This apparatus includes aseries of wash sprayers 134 (disposed in a top panel 136) and aperforated plate 138 in the support 140. Wash liquid passes through veryfine holes 142 in the plate 138 to facilitate drainage of liquid fromthe froth 144 without substantial loss of froth 144 from the support140. A bottom channel 146 formed by a bottom panel 150 and side panels152 and 154 of the support 140 collects the wash water for return to theflotation cell (not shown), directly or indirectly, or for otherdisposal. The apparatus is provided with an air exit opening 156, andoverflow weir 158 that leads to a launder 160. The top panel 136 canextend to the vicinity of the overflow weir 158 or, in some embodiments,can extend only so far as to provide support for the wash sprayers 134.

[0095]FIG. 9 shows the interface of an embodiment of a froth cleaningapparatus 162 according to the disclosure with a type of flotation cell164 having an overflow weir 166 and launder 167 around the circumferenceof the cell 164. The apparatus 162 is shown before installation (inphantom lines) and after installation. The apparatus 162 generallyincludes a hood 168 including a lower peripheral edge 170 for interfacewith the top of the cell 164 and a top 186. The apparatus 162 alsoincludes a discharge orifice 172 in the hood 168 that leads to acleaning section 174 having wash sprayers. 176, an air exit opening 178,an overflow weir 180, and a launder 182. The cleaning section 174 cantake any configuration, such as those described in FIGS. 1-8. Uponinstallation of the apparatus 162, the weir 166 and launder 167 aresealed to prevent leakage of pulp 184, and instead the washed froth 188passes to launder 182.

[0096] To provide clearance for the hood, apparatus such as a pulley 190that drives an agitator 192 might need to be relocated higher on theshaft 194 of the agitator 192, as shown in the figure wherein the formerposition of the pulley 190 is shown in phantom lines 198. Though notshown, the apparatus could be configured to make use of the existinglaunder 167 by conveyance of the washed froth 188 thereto.

[0097]FIG. 10 is a partial view of a froth cleaner section 200 includinga froth support 202 having a movable upper section 204 to control thedepth of froth 208 passing through the cleaner 200. In such anembodiment, wash sprayers 210 can be connected to associated flexiblewash liquid supply lines 212 (e.g., hoses) to permit repositioning ofthe upper section 204.

[0098]FIG. 11 shows a partial cutaway view of a froth support apparatus220 that includes a repositionable panel 222 to change the height of apassage formed therewith from the bottom to control the depth of frothpassed through the passage. As shown, the panel 222 can be secured inone of a selection of grooves 224 provided in side walls 228 (one shown)of the support 220. The panel 222 is also shown with a lip 230 to directpulp and/or froth above the panel 222.

[0099]FIG. 12 shows a froth flotation apparatus according to thedisclosure that includes a froth cleaning section 240 around thecircumference of the major body of a cell 242 and a hood 244 to allow alarger volume for froth cleaning. Such a cleaning section 240 and hood244 can also be independently adapted for interface with an independentflotation cell, analogous to the apparatus described and shown in FIG.9.

[0100] FIGS. 13 to 15 show more apparatus adapted to cope with potentialintermittent surges of pulp and concomitant or resulting changes in theposition or level of the liquid froth interface. FIG. 13 shows a partialcutaway view of a froth support apparatus 246 that includes a mechanicalfroth motivator 250 in the form of a spindle 252 with pushers 254. Thepushers 254 in this embodiment preferably intersect with the support 246at the bottom panel 256 in a substantially sealing fashion, such thatonly insubstantial amounts of pulp and/or froth are permitted to passthe motivator 250 through the bottom passage bounded by the spindle 252,the bottom panel 256, and side walls 260 and 262 of the support 246 in adirection counter to the rotation of the froth motivator 250 (see alsoFIG. 14). For example, pushers 254 can be constructed of a flexiblematerial and in a length slightly greater than the distance to thebottom panel 256 from the spindle 252, such that a pusher 254 bends uponintersection with the bottom panel 256, for example when perpendicularto the bottom panel 256. In one embodiment, the motivator 250 isdesigned or disposed to leave an open pathway between the top panel 264of the support apparatus 246. In another embodiment, the motivator 250is designed or disposed such that a pusher 254 intersects with the toppanel 264 of the support apparatus 246. One or more wash sprayers 266preferably are disposed at a location upstream of the froth motivator250.

[0101] The a froth flotation apparatus (not shown) can be operated withsuch a support apparatus 246 in such a manner that the pulp level 270 isbelow the froth motivator 250, as shown, or the pulp level can be raisedsuch that at least a portion of the motivator 250 intersects the pulplevel 270. In the alternative, the motivator 250 can be disposed at alocation in the support apparatus 246 at a location closer to theflotation cell, such that it at least partially intersects the pulplevel 270, wherein the pulp level 270 is similar to that shown in FIG.13.

[0102] If the pulp level 270 intermittently rises in surging fashion,such a motivator 250 can aid in preventing the pulp 272 fromcontaminating the cleaned froth 274 or from spilling over the weir 276into the launder 278. Air exits through opening 280.

[0103]FIG. 15 shows a cross-section of a froth flotation apparatus 282according to the disclosure that includes a generally zigzag-shapedfroth cleaning section 284. The cleaning section 284 includes a firstsupport section 286 disposed at a first upwardly-inclined angle β withrespect to the major plane of a base 290 and a second support section292 disposed at a second upwardly-inclined angle γ with respect to themajor plane of the base 290. The first and second sections 286 and 292are disposed in opposing directions with respect to a planeperpendicular to the major plane of the base (e.g., a plane parallel tothe side wall 294). A wash sprayer 296 is disposed in the second supportsection 292 to spray wash liquid on the froth therein.

[0104] The apparatus 284 includes a third support section 300 disposedat a downwardly-inclined angle δ with respect to the base 290 of theflotation apparatus 282 for flow of the cleaned froth into a launder302. A vent 304 for discharge of air 306 is in fluid communication withthe third section 300, and can be located at any point along the thirdsupport section 300. The launder 302 can have an open top (e.g., at thelower end of the third support section 300, as illustrated) fordischarge of air in addition to, or instead of a vent 304.

[0105] If the pulp level 310 intermittently rises in surging fashioninto the first support section 286, the non-linear design of the firstand second support sections 286 and 292 can break the momentum of thepulp 312 and aid in preventing the pulp 312 from contaminating thecleaned froth 314 or from spilling over into the launder 302.

EXAMPLES

[0106] The following examples are provided to illustrate the inventionbut are not intended to limit the scope of the invention.

Example 1

[0107] A froth cleaning apparatus according to FIG. 16, was constructedfor interface with a 2-inch MICROCEL flotation column (developed byRoe-Hoan Yoon of Virginia Polytechnic Institute and State University).The froth cleaning apparatus had a connecting conduit 320 and a washingconduit 322. The washing conduit 322 was 410 mm in length and 25 mmsquare in cross-section, with two internal grooves 324 on the lower face328, as shown in FIG. 17.

[0108] A series of 3 wash sprayers (two shown, elements 330 and 332)were disposed in series along the path of froth flow, and anycombination of the sprayers could be used for dispensing wash fluid. Inthe experiments reported in Examples 2 to 4 below, only two washsprayers were used. The first wash sprayer 330 was disposed 165 mm fromthe outlet end of the wash conduit 322 and the second wash sprayer 332was disposed 100 mm from the outlet end of the wash conduit 322.

[0109] The froth cleaning apparatus was attached to the main body of theMICROCEL column using a series of connectors and elbows (replacing thetop launder section of the conventional MICROCEL column) to facilitateeasy manipulation of the washer inclination at different angles. Theresults reported in Examples 2-4 were obtained by operating theapparatus at an angle of 22.5 degrees.

Example 2

[0110]FIGS. 18 and 19 are charts comparing sulfur and ash rejectioncapabilities, respectively, of the MICROCEL flotation column with aconventional froth washer and the MICROCEL flotation column equippedwith a froth cleaning apparatus according to Example 1 (two washsprayers in operation). The results were obtained from treatment of −48mesh particle size fraction Pittsburgh No. 8 coal refuse. The curves inFIGS. 18 and 19 represent predictions based on advanced flotationrelease tests, described below.

[0111] The results show that the column equipped with the apparatus ofExample 1 generated a coal product with an ash content as low as 8.0% ata combustible recovery level of 80%, compared with an ash content of 10%at the same combustible recovery level achieved by the conventionalflotation column and froth-washing system (see FIG. 18).

[0112] The results also show that the column equipped with the apparatusof Example 1 provided superior separation performance in terms of sulfurrejection. At a combustible recovery of nearly 80%, the column equippedwith the apparatus of Example 1 reduced the total sulfur content tolevels ranging from 5.1% to 3.3%, whereas 4.1% was the lowest sulfurconcentration that could be achieved by the flotation column with aconventional froth washing system at the same recovery value (see FIG.19).

Example 3

[0113]FIGS. 20 and 21 are charts comparing ash and sulfur rejectioncapabilities, respectively, of the same apparatus used in Example 2, andfrom treatment of the −325 mesh particle size fraction of the samesample as used for Example 2.

[0114] These side-by-side tests (see FIG. 20) again show a superiorsulfur rejection performance for the column equipped by the apparatus ofExample 1 (1.75% total sulfur content in the product) as compared to theconventional flotation column (2.0% total sulfur content in theproduct). The performance of the apparatus of Example 1 also appeared tobe superior for achieving a low ash product (better combustiblerecovery), but for higher ash products the conventional column providedbetter combustible recovery (see FIG. 21).

Example 4

[0115] Carrying capacity tests were carried out to determine the effectof pulp feed rate (g/(min·cm²)) on the rate of combustibel recovery(g/(min·cm²)) feed) of a flotation column modified according to thedisclosure. FIG. 22 shows the ability of an apparatus modified accordingto the disclosure to produce a larger amount of product per minute, on aconstant cross-sectional area of column basis, believed to be the resultof reduced bubble coalescence. As shown in FIG. 22, for the −325 meshparticle size fraction of coal refuse from the Pittsburgh No. 8 CoalSeam, the carrying capacity of the flotation column modified accordingto the disclosure was found to be 28% greater than the conventionalflotation column.

Example 5

[0116] Various tests were carried out with a rectangular cuboid pilotplant-sized (about 37 liters in volume, outer dimensions about 28 cm by43 cm by 51 cm tall) subaeration cell equipped with an apparatus similarto that shown in FIG. 9, referred to herein as the “Big Cell.” Asdescribed above, subaeration cells are a proven technology with a higherthroughput than other commercial flotation systems, and are moreversatile and flexible than any other commercial flotation system.Adaptation of the subaeration cell by installing an apparatus accordingto the disclosure eliminates many of the shortcomings of the subaerationcell, such as incomplete separation of the product from the pulp.

[0117] Advanced Flotation Release (AFR) analysis should represent thebest results that can be obtained with any commercial flotation device.The AFR analysis is a process developed by Southern Illinois Universityto establish the limits of achievement of flotation devices with regardsto combustible recovery and reduction of ash-forming mineral matter andsulfur. A graph produced according to AFR analysis is established byfloating the coal in a typical subaeration cell under conditions thatwill guarantee the fullest recovery of all the combustible as a product.This product is then floated in a packed column under optimum conditionsand the material floated within a certain time interval is collected andanalyzed.

[0118] These tests conducted with the Big Cell, on feeds containingvarious amounts of ash and sulfur, demonstrated ash and sulfur rejectionrates by the Big Cell better than the level predicted by AFR analysis.

[0119] Thus, FIG. 23 plots combustible recovery versus total ash contentof the product separated from a slurry (5% solids) of Illinois fine coal(8% ash) by AFR analysis and with the Big Cell. The figure shows thatthe Big Cell achieved about 80% combustible recovery with as low as 4%ash content in the product, which is better than approximately 70%combustible recovery predicted by AFR analysis.

[0120]FIG. 24 plots combustible recovery versus total sulfur content ofthe product separated from a slurry (5% solids) of Illinois fine coal(3.17% sulfur) by AFR analysis and with the Big Cell. As shown in thefigure, the Big Cell achieved much lower sulfur content in the productthan that predicted by AFR analysis for the same levels of combustiblerecovery.

[0121]FIG. 25 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids) of a reconstituted feed (24%ash) by AFR tests and with the Big Cell. In these tests, for this feed,the Big Cell approached the theoretical maximum ash rejection ofconventional devices predicted by the AFR analysis for a range ofcombustible recovery levels.

[0122]FIG. 26 plots combustible recovery versus total sulfur content ofthe product separated from a slurry (5% solids) of a reconstituted feed(3.44% sulfur) by AFR tests and with the Big Cell. As shown in thefigure, the Big Cell achieved much lower sulfur content in the productthan that predicted by AFR analysis for the same levels of combustiblerecovery.

[0123]FIG. 27 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids) of high-ash Illinois coalrefuse (40% ash) by AFR tests and with the Big Cell. As shown in thefigure, the Big Cell achieved higher-than-predicted combustible recoverylevels for higher ash content levels.

[0124]FIG. 28 plots combustible recovery versus total sulfur content ofthe product separated from a slurry (5% solids) of Illinois coal refuse(3.7% sulfur) by AFR tests and with the Big Cell. As shown in thefigure, the Big Cell achieved a higher combustible recovery level in theproduct than that predicted by AFR analysis for the same levels ofsulfur content in the product.

[0125]FIG. 29 plots combustible recovery versus total ash content of theproduct separated from a slurry (5% solids; 24% ash) of fine coal mixedwith washing plant rejects (tailings) by AFR tests and with the BigCell. Plotted for comparison are data obtained from in-plant tests withthe Big Cell fed a slurry with 3% solids and 26% ash content extractedfrom the tailings outfall from a coal washing plant in Illinois.

[0126] Various embodiments of the inventions can provide differingadvantages, based on the objectives desired to be achieved with aflotation operation. For example, an embodiment of the invention adaptedto retrofit a single subaeration cell can produce a cleaner coal productthan that achieved with a flotation column, but at the large throughputrate of a common subaeration cell. Thus, a single device can take theplace of a system of subaeration cell batteries in which the froth ortails from one cell are re-cleaned in a subsequent cell to generate aproduct of desired purity. An apparatus according to the disclosure canbe operated to produce a well-drained, dry froth or a relatively wet,water-laden froth, depending on the needs of the system and the capacityof an associated filtration system.

[0127] The foregoing description is given for clearness of understandingonly, and no unnecessary limitations should be understood therefrom, asmodifications within the scope of the invention may be apparent to thosehaving ordinary skill in the art. For example, variables such asaeration gas velocity, bubble size, pulp temperature, impeller speed,collector dosage, frother dosage, feed percent solids, volumetric feedrate, washer inclination, number of wash sprayers, froth height, biasrate, aeration gas volumetric rate, wash fluid volumetric rate, washfluid velocity, wash fluid frother addition, and other properties knownto those skilled in the art and based on the disclosure herein, can becontrolled within desired ranges to affect the quality and speed ofseparation.

What is claimed is:
 1. A process of separating a desired constituentfrom a mixture of particulate matter, comprising the steps of:conditioning a liquid mixture of particulate matter comprising a desiredconstituent with a frothing agent to create a pulp; aerating said pulpto generate a float fraction of froth supported on the surface of anon-float fraction of pulp, said float fraction comprising a pluralityof bubbles, at least a portion of said bubbles selectively attached tosaid desired constituent of said pulp; separating a portion of frothfrom said float fraction; draining said separated froth; washing saidseparated froth with a liquid to dislodge particles comprising one ormore of non-selectively attached, entrained, and entrapped particles;and recovering said washed froth.
 2. The process of claim 1, comprisingdraining said froth before washing said froth.
 3. The process of claim1, wherein one or more of said separating step and said draining stepcomprises forcing said froth up an incline.
 4. The process of claim 1,wherein said desired component is coal.
 5. The process of claim 1,further comprising the step of conditioning one or more constituents ofsaid mixture of particulate matter to render said constituenthydrophobic or more hydrophobic.
 6. The process of claim 1, wherein saidaeration step comprises adding air to said pulp.
 7. The process of claim1, wherein said liquid mixture of particulate matter is aqueous and saidwashing step comprises washing said separated froth with a componentselected from water, a froth modifier, and combinations thereof.
 8. Theprocess of claim 1, further comprising the step of substantiallyseparating said wash liquid comprising dislodged particles from contactwith separated froth.
 9. The process of claim 8, comprising the step ofsubstantially separating said wash liquid comprising dislodged particlesfrom contact with said washed froth.
 10. The process of claim 1, whereinthe average particle size of said mixture is less than about 0.65 mm.11. The process of claim 1, further comprising the step of controllingthe depth of said separated froth.
 12. The process of claim 11, whereinsaid controlling step comprises passing said separated froth through apassage having a controlled height.
 13. A froth flotation apparatus forseparating a desired constituent from a mixture of particulate matter,comprising: a wall defining a flotation cell; an aerator for aerating amixture of particulate matter to produce froth; a feed opening forintroducing a mixture of particulate matter and/or froth into said cell;a discharge orifice in a wall of the cell; a froth support incommunication with said discharge outlet for receiving and supportingsaid froth; and a wash sprayer disposed upstream of said dischargeorifice.
 14. The apparatus of claim 13, comprising a froth depthcontroller.
 15. The apparatus of claim 14, wherein said froth depthcontroller is selected from the group consisting of said dischargeorifice, said froth support, and combinations thereof.
 16. The apparatusof claim 13, wherein said froth support comprises a bottom interiorsurface of a conduit having an average width greater than an averageheight.
 17. The apparatus of claim 13, further comprising a base havinga major plane and wherein said froth support is disposed at anupwardly-inclined angle with respect to said major plane of said base.18. The apparatus of claim 13, wherein at least a portion of said frothsupport is disposed above said flotation cell.
 19. The apparatus ofclaim 18, wherein said froth support has a convoluted path for fluidtravel.
 20. The apparatus of claim 19, wherein said froth supportcomprises a first support section disposed at a first upwardly-inclinedangle with respect to said major plane of said base; and a secondsupport section disposed at a second upwardly-inclined angle, same ordifferent, with respect to said major plane of said base, wherein saidfirst and second sections are disposed in opposing directions withrespect to a plane perpendicular to said major plane of said base. 21.The apparatus of claim 13, wherein said froth support comprises a frothdrainer to substantially separate wash fluid containing dislodgedparticles from contact with one or more of separated froth and washedfroth.
 22. The apparatus of claim 21, wherein said froth drainercomprises a perforated region of said froth support.
 23. The apparatusof claim 13, comprising a plurality of wash sprayers.
 24. The apparatusof claim 13, wherein said support comprises a vent.
 25. The apparatus ofclaim 13, further comprising a screen disposed in proximity to and/orupstream of said discharge orifice in the path of travel of at least aportion of the generated froth.
 26. The apparatus of claim 13, furthercomprising a froth motivator to assist in conveyance of said frothupstream of said discharge orifice.
 27. A froth cleaning apparatus foruse with a froth flotation cell that causes froth to collect at the topof the froth flotation cell, said apparatus comprising: a hoodcomprising a lower peripheral edge for interface with the top of a frothflotation cell; a discharge orifice disposed in said hood; a frothsupport in communication with said discharge outlet for receiving andsupporting said froth; and a wash sprayer disposed upstream of saiddischarge orifice.
 28. The apparatus of claim 27, comprising a frothdepth controller.
 29. The apparatus of claim 28, wherein said frothdepth controller is selected from the group consisting of said dischargeorifice, said froth support, and combinations thereof.
 30. The apparatusof claim 27, wherein said froth support comprises a bottom interiorsurface of a conduit having an average width greater than an averageheight.
 31. The apparatus of claim 27, wherein said lower edge liessubstantially in a major plane and wherein said froth support isdisposed at an upwardly-inclined angle with respect to said major plane.32. The apparatus of claim 31, wherein said froth support has aconvoluted path for fluid travel.
 33. The apparatus of claim 32, whereinsaid froth support comprises a first support section disposed at a firstupwardly-inclined angle with respect to said major plane; and a secondsupport section disposed at a second upwardly-inclined angle, same ordifferent, with respect to said major plane, wherein said first andsecond sections are disposed in opposing directions with respect to aplane perpendicular to said major plane.
 34. The apparatus of claim 27,wherein said froth support comprises a froth drainer to substantiallyseparate wash fluid containing dislodged particles from contact with oneor more of separated froth and washed froth.
 35. The apparatus of claim34, wherein said froth drainer comprises a perforated region of saidfroth support.
 36. The apparatus of claim 27, wherein said supportcomprises a vent.
 37. The apparatus of claim 27, comprising a pluralityof wash sprayers disposed upstream of said discharge orifice.
 38. Theapparatus of claim 27, further comprising a screen disposed in proximityto and/or upstream of said discharge orifice in the path of travel of atleast a portion of the generated froth.
 39. The apparatus of claim 27,further comprising a froth motivator to assist in conveyance of saidfroth upstream of said discharge orifice.